1. Filed of the Invention
The present invention concerns a method and an arrangement for filling a silo.
2. Description of Related Art
In industries in which large quantities of fluidizable material are handled, for example oxide for the aluminium industry, segregation problems arise with the current systems. Such segregation means that local variations can arise with regard to the accumulation of the material""s lower (finer) and upper (coarser) fractions. The segregation is initiated during the filling operation, in which large movements of air can easily occur in the silo with consequent accumulation of finer fractions, depending on the geometry of the silo, particularly out by the silo""s walls. During emptying or removal of mass from the silo, there can subsequently be problems with varying fractions, which, in turn, can also result in operational disturbances downstream of the silo such as blockages in the silo and accompanying transport systems. Operational disturbances in the silo itself or downstream of the silo can result in an increased possibility of discharge of fine dust to the environment. In connection with the production of aluminium, such operational disturbances can result in undesired situations in the cells with sludge formation, anode damage, blockage of point feeders, etc. so that the cells have to be xe2x80x9copenedxe2x80x9d to implement various measures, with an increased risk of the emission of heat and fluoride gases to the environment.
DE-U1-9408840 discloses a storage silo having a material distributor centrally located at the top part of the silo. The distributor comprises a container with a centrally top-mounted inlet through which material is fed into the container, and a centrally arranged outlet in its bottom together with outlets in its sides. The outlets in the sides of the container are connected with air slides for the transport of materials to inlet openings arranged in the top part of the silo. Inside the container there is arranged a ring member below the centrally top-mounted inlet. The ring member is of limited vertical extension allowing materials to flow both beneath its lower edge and above its upper edge to the outlets in the sides of the container. The centrally arranged outlet is operated by a closing valve while transport out of the outlets can be operated by activating the air slides by fluidizing gas. By filling the silo in an alternating manner either through the centrally arranged outlet of the container or through the outlets in the sides of the container via the air slides to the inlet openings in the top part of the silo, batches of material of different qualities can be distributed over the cross-sectional area of the silo thus resulting in a mixing of different fraction sizes in the filling procedure of the silo. However, problems related to air induced segregation will still be present in this solution, in particular under filling operations at low material levels in the silo, i.e. under conditions where the falling height of the material will be high.
Attempts are usually made to avoid operational disturbances as a result of air-induced segregation in a silo by not emptying the silo completely. During operation, such measures mean that the problems associated with segregation are postponed as the silo gradually accumulates so much fine material that it must come out in one way or another. The options for removing it are then either to empty the silo completely and remove the fine material or to use it in the process with the risk of operating problems.
The following article, xe2x80x9cAn anti-segregation tube to counteract air current segregationxe2x80x9d, by Are Dyrxc3x8y and Gisle G. Enstad, pages 27-30, POSTEC Newsletter No. 16, December 1997, proposes a method for counteracting segregation during the filling of a silo using a central pipe (filling pipe) which extends from the top of the silo down to its bottom. The pipe is equipped with valves along its full length which are closed until filling begins. When the silo is filled, the lowest valve opens because the pressure inside the pipe is greater than that outside the pipe. In connection with the upper part of the pipe, it is possible to mount an inlet lock to prevent air accompanying the material with which the pipe is filled and to regulate the quantity fed into the pipe. This solution has proved to be effective in counteracting segregation when filling a silo.
A restriction of the arrangement described above is that the pipe, which is mounted centrally in the tank, will be exposed to large forces from the material with which the silo is filled. A further restriction is that it is difficult to use the full capacity of the silo with this arrangement as the angle of slip of the material will prevent complete filling out by the walls of the silo. Moreover, there may be a need for additional deaeration with the proposed system in order to avoid segregation on account of high air speeds during filling, in particular in connection with the handling of material which has previously been fluidized, i.e. material which contains air from prior upstream transport.
The present invention attempts to avoid the above problems. The present invention is based on the quantity of air which is fed into the silo during filling being kept to a controlled, minimal level. The present invention also causes the actual material which is fed into the silo to be distributed in a favorable manner and allows the silo to be filled more quickly. Furthermore, the present invention represents an arrangement which is more robust with regard to the stresses caused by large material movements such as slips in the silo.